1. Field of the Invention
The present invention relates to an optical encoder which is utilized for measuring positions of machine tools and semiconductor manufacturing systems.
2. Description of the Prior Art
An optical encoder comprises a light emitting unit which emits a light beam and photoelectric converting elements which are disposed behind two diffraction gratings. The optical encoder and detects light having passed through the two diffraction gratings with the photoelectric converting elements, thereby detecting a moving distance on the basis of a variation of a light intensity caused due to a relative movement between the two diffraction gratings.
Generally speaking, a diffraction grating is composed of transmissive portions allowing transmission of the light and non-transmissive portions not allowing transmission of the light which are arranged alternately. An arrangement pitch of the two portions is referred to as a grating pitch, and each of the transmissive portions and the non-transmissive portions has a width equal to 1/2 of a grating pitch P.
In an encoder which uses two diffraction gratings, a transmission light amount has a miaximum value and an output detected with the photoelectric converting element also has a maximum value when the transmissive portions of the gratings are matched with each other. Further, the transmission light amount has a minimum value and the output detected with the photoelectric converting elements also has a minimum value when the transmissive portions of one of the diffraction grating are matched with the non-transmissive portions of the other diffraction grating. An electric signal outputted from the photoelectric converting element varies between the maximum value and the minimum value dependently on relative displacements between the two diffraction gratings. When the two diffraction gratings are displaced at an equal speed relative to each other, an output signal obtained with the photoelectric converting element should ideally be a triangular wave signal having a period P. In actuality, however, the triangular wave signal is distorted under influences due to diffraction and so on. The detection of relative displacements has conventionally been carried out while regarding the triangular wave signal as a sinusoidal wave.
Furthermore, the optical encoder which has the conventional configuration uses a second grating 2 such as that shown in FIG. 1 as a second diffraction grating (hereinafter referred to as a "second grating") which is to be disposed behind a first diffraction grating (hereinafter referred to as a "first grating"). Since different phases are detected at different locations which are apart from one another on the first grating for detecting a position, the optical encoder has a problem that it allows signals to be unbalanced under influences due to stains, damage or errors on the scale, thereby producing measuring errors. The optical encoder further has another problem in that it allows signals of the different phases to be varied due to variations of parallelism or intensity of a light beam from a light source.
For preventing crosstalk between lights having different phases, it is necessary to reserve spaces between each phase of photo detecting devices on the second grating and a light receiving section, thereby enlarging the optical encoder.
For solving the above problems, Japanese Patent Application Laid-open No. 8-201117 discloses an optical encoder in which light having passed through a first grating is received directly with photo detecting devices arranged in a shape of a grating. In this optical encoder, the photo detecting devices are arranged in a light receiving section 3 so as to form a grating as shown in FIG. 2. Since the photo detecting devices for different phases are mixed in the shape of the grating on photoelectric converting elements, the optical encoder is capable of detecting the different phases at positions which are substantially the same. Therefore, the optical encoder is capable of detecting displacements with a higher accuracy than the optical encoder which uses such the second grating 2 such as that shown in FIG. 1 even when signals are under influences due to stains, damage or errors on a scale or when parallelism or intensity of a light beam from a light source is varied.
When the stains, damage and errors are caused on the scale at intervals which are nearly equal to a pitch of scales, however, signals of one phase only are influenced by the stains, damage or errors, thereby causing errors in detection of the positions.
In the light receiving section 3 shown in FIG. 2, the photo detecting devices are arranged not at intervals which are definite but with predetermined phase differences so as to eliminate higher harmonic components of high orders such as third and fifth orders for precise positional detection with distorted components eliminated. For eliminating the distorted higher harmonic components, for example, of the third and fifth orders, however, it is necessary to arrange at least four photo detecting devices. Though a sufficient averaging effect is exhibited regardless of uniformity of an illumination light beam when the photo detecting devices are arranged in a sufficiently large number, there is posed a problem that the averaging effect is lowered and the capability to eliminate the distorted higher harmonic components of the third and fifth orders is degraded when a grating has a small number of slits, for example, four to several slits.